Wakeup, you need to separate the biology from the sociology. The OP is asking why he is attracted to young teenagers. Its b/c they are young and fertile looking.
Sure, our society *Western society*, lets be clear, keeps young women home longer and educates them. This is a good thing for society, IMO. Men know that sex with our teenage girls is taboo, so they keep such thoughts to themselves or condition themselves to just not go there.
So, I say its natural to think these thoughts provided the OP isn't fantasizing about teenagers who still look like little girls. But, I see the girls at my son's school (13-14) and some of them are scarily adult-looking. Thank you Brittany Spears. Emphasis on the looks part, the mind is years behind.
Second thoughts can generally be amended with judicious action; injudicious actions can seldom be recovered with second thoughts.
--Cyteen by C.J.Cherryh